Conversion coatings for titanium, aluminum, or other metals are electrolytic or chemical films that promote adhesion between the metal and an organic adhesive resin, especially for adhesive bonding. Anodizing is a conventional process for making electrolytic films by immersing titanium or its alloys in chromic acid or an alkaline earth hydroxide or aluminum in chromic, sulfuric, or phosphoric acid. Anodizing produces a porous, microrough surface into which primer (a dilute solution of adhesive) can penetrate. Adhesion results primarily from mechanical interlocking between the rough surface and the primer. Chemical films include either a phosphate-fluoride conversion coating or films made with alkaline peroxide or other alkaline etchants for titanium substrates and Alodine films (i.e., a chromate conversion coating) for aluminum substrates.
Because they use strong acids or strong bases and toxic materials (especially heavy metals such as chromates), these surface treatment processes are disadvantageous from an environmental viewpoint. They require significant amounts of water to rinse excess process solutions from the treated parts. The rinse water and spent process solutions must be treated to remove dissolved metals prior to their discharge or reuse. Removing the metals generates additional hazardous wastes that are challenging to cleanup and dispose. Controlling exposure of workers to the hazardous process solutions during either tank or manual application requires special control and exposure monitoring equipment that increases the process cost. They greatly increase the cost of using the conventional wet-chemical processes. A process that will produce adhesive bonds with equivalent strength and environmental durability to these standard processes without generating significant hazardous wastes while eliminating the use of hazardous or toxic materials would greatly enhance the state-of-the-art. The present invention is one such process. In addition, the process of the present invention can be applied by spraying rather than by immersion. Therefore, it is more readily used for field repair and maintenance.
Surface anodizing chemically modifies the surface of a metal to provide a controlled oxide surface morphology favorable to receive additional protective coatings, such as primers and finish paints. The surface oxides function as adhesion coupling agents for holding the paint lacquer, an organic adhesive, or an organic matrix resin, depending on the application. Anodizing improves adhesion between bonded metals. It also improves adhesion between the metal and a fiber-reinforced composite in hybrid laminates, like those described in U.S. Pat. Nos. 4,489,123 or 5,866,272. We incorporate these patents by reference. Structural hybrid laminates have strengths comparable to monolithic metal, and have better overall properties than the metal because of the composite layers. At higher temperatures (like those anticipated for extended supersonic flight), conventional anodized treatments have inadequate performance in addition to being environmentally unfriendly. The thick oxide layers that anodizing produces become unstable at elevated temperatures. The oxide layer dissolves into the base metal to produce surface suboxides and an unstable interfacial layer.
Obtaining the proper interface for the organic resin at the surface of the metal is an area of concern that has been the focus of considerable research. For example, cobalt-based surface treatments for aluminum are described in U.S. Pat. Nos. 5,298,092; 5,378,293; 5,411,606; 5,415,687; 5,468,307; 5,472,524; 5,487,949; and 5,551,994. U.S. Pat. No. 4,894,127 describes boric acid-sulfuric acid anodizing of aluminum.
Bonding sites on surfaces for binders include covalent bonds, hydrogen bonds, or van der Waals forces. Conventional approaches (anodizing and chromate conversion coating) promote adhesion by producing a high surface area coating which has both mechanical and physical (Lewis acid-base, dispersion, hydrogen bonding, etc.) interactions with the adhesion primer. An aerospace coupling agent can be used to create strong covalent bonds between the metal substrate and the organic primer. The present invention improves adhesion by crating a sol-gel-based coating containing a coupling agent on the metal surface. A metal-to-resin gradient occurs through a monolayer of organometallic coupling agents. Generally we use a mixture of coupling agents. The organometallic compounds preferably have zirconium or silicon active moieties to interact with, react with, or bond to the metal surface. Some mechanical interaction may result from the surface porosity and microstructure. The organic portion of the organometallic compounds usually has a reactive functional group appropriate for covalently bonding with the adhesive or matrix resin. A preferred sol-gel film is made from a sol having a mixture of organometallic coupling agents. One component (usually containing zirconium) bonds covalently with the metal while a second component bonds with the resin. Thus, the sol-gel process orients the sol coating having a metal-to-resin gradient on the surface.
The standard anodizing processes, conversion coatings, or oxide surface preparations, especially for titanium, are inappropriate to use with new polyimide adhesives that are promising as adhesives for vehicles that will experience extended exposure to hot/wet conditions. At high temperatures, the solubility of oxygen in titanium is high and the oxide layer simply dissolves with the oxygen migrating into the base metal. The result is interfacial failure at the metal-adhesive interface. To alleviate this type of bond failure, the surface oxygen needs to be tied up in a stronger bond that will not dissociate in bonding or during operation of the system. A zirconate-silicate sol coating of the present invention is useful at these extended hot/wet conditions because the Zr—O bond that forms between the coating and the metal surface is more durable than a Ti—O bond. The free energy of formation for the metal oxides is such that a Zr—O bond is more stable at high temperatures than a Ti—O or Si—O bond. The higher bond strength of the Zr—O bond prevents dissolution of the oxide layer, so the Zr component in our sol coating functions as an oxygen diffusion barrier. We can use yttrium, cerium, or lanthanum in addition to or as a replacement for the Zr, because these elements also produce high strength oxide bonds that function as an oxygen diffusion barrier. The high cost of these compounds, however, dictates that they be used sparingly. Therefore, we developed a mixed metal coating having Zr and Si to produce the desired metal-to-resin gradient needed for good adhesion in structural adhesive bonds, hybrid laminates, or paint adhesion applications. Our coating integrates the oxygen diffusion barrier function of the Zr (or its alternatives) with an organosilicate network desirable for superior bonding performance.
The present invention combines pigments with the sol-gel corrosion inhibitor thin film, to overcome many of the shortcomings of paint. Paints are commonly used to protect a surface and to provide color, gloss, reflectivity, or the like on a substrate. Paints generally disperse metal or ceramic pigments and a binder in a water or organic vehicle to form a film when dried on a surface. Typically, the binder is an organic resin. Paints generally have application only at relatively low temperatures. They can be difficult to apply uniformly. They are relatively heavy and are expensive to repair. Extreme environments, such as high temperatures or space environments with high ultraviolet, atomic oxygen, and particle exposure, degrade typical organic resins. The present invention combines pigments with sol-gel binder thin films to give greater performance and durability under extreme conditions.